Conventionally, transmissive-type and semi-transmissive-type liquid crystal display devices comprise a liquid crystal display panel and a backlight for supplying light transmitting to the liquid crystal panel.
Generally, a backlight includes a light source and a light guide plate, and a small fluorescent tube called CCFL (cold cathode fluorescent tube) is used as the light source. A first principal surface (front surface) of the light guide plate is disposed so as to correspond to the display area of the liquid crystal display panel, and a diffusion area for diffusing and reflecting light toward the front surface side is provided on a second principal surface (back surface) thereof opposite to the aforementioned principal surface.
The CCFL light source is disposed at an end surface of the light guide plate, and light of CCFL incident on the end surface of the light guide plate is transmitted into the light guide plate, and is diffused/reflected on the back surface side of the light guide plate to be directed from the light guide plate toward the liquid crystal display panel. Thus, the light source is converted from a linear light source into a homogeneous planar light source to be utilized as the light source for the liquid crystal display device.
However, this CCFL light source uses Hg (mercury) encapsulated in a discharge tube, and ultraviolet rays emitted from mercury excited by electrical discharge strike the fluorescent substance on the CCFL tube wall to be converted into visible light rays. For this reason, when considering the environmental aspect, using an alternative light source is required for restricting the use of hazardous mercury. In addition, in order to illuminate the CCFL, a high-voltage and high-frequency switching circuit is necessary. However, since this generates high frequency noise, not only noise prevention is additionally required, but also problems such as slow light-up under low temperature and the like are prone to arise.
In the meantime, as a new light source, a backlight utilizing a light emitting diode (LED) module storing light emitting diode chips characterized by a point light source has been proposed.
In tune with the recent trends toward lower prices, higher luminous efficiency and environmental regulations, this backlight (LED backlight) using the light emitting diode module is becoming widely used as the backlight for liquid crystal display panels.
With increased brightness and size (of the display area) of liquid crystal display devices, arranging a plurality of light emitting diode modules in an LCD device can be taken into consideration.
Accordingly, when an LED backlight is used for a high brightness, large size liquid crystal display panel, the light emitting diode module, which is a point light source, needs to be converted into a planar light source that emits light rays evenly (light source that has been converted into homogeneous light rays at the light-emitting surface of the light guide plate). For this reason, it is necessary to adjust the material and structure of the diffusion area of the back surface of the light guide plate, as well as to dispose the light emitting diode module at an optimum position in accordance with the optical characteristics thereof.
The greatest problem here is lifetime of the light emitting diode module. The warranty life (half-life of luminance) of a conventional, commonly used CCFL at a tube current of 5 mA is 50000 hours at an ambient temperature of 25° C., and 45000 hours at an ambient temperature of 60° C. On the other hand, as for the lifetime of an LED chip, for example, in the case of a top-view type LED (NSCW455) produced by Nichia Corporation, estimated lifetime data (half-life of luminance) at forward current, F=20 mA, is about 12000 hours at an ambient temperature of 25° C., and only about 5500 hours at 50° C. It is obvious that lifetime of light emitting diode module is shorter than that of CCFL.
Because of many technical difficulties, it would be unrealistic to expect a light emitting diode module to have a lifetime equivalent to that of CCFL at this time.
It is therefore necessary to provide a light emitting diode module used as a backlight for a liquid crystal display device with a replaceable structure.
As one example of prior art related to the mounting of a light emitting diode module, there is a planar light source apparatus disclosed in Japanese Unexamined Patent Publication NO. 2001-184924.
Patent document 1: Japanese Unexamined Patent Publication NO. 2001-184924